Most I spent at Arsenal was £3000 for 4 seats in a box for the Boxing Day game against the Spuds about 8-9 years ago. All for a children's charity and we won so all good!
You can get Club Level tickets for £90+. I like these; they're good seats above the main stands and decent bars/restaurants before and after the game to boot.
The thing with Arsenal is that it's so easy to get to from the City/ West End. Combined with the attractive football it's one of the obvious entertainment venues for business in Town. Demand is high and the prices aren't unreasonable for someone on a London Salary especially if your company is paying.